The plaintiff would have to prove that the person who wrote it intended the reader to understand that she was literally a member of the Nazi party and that it wasn't simply hyperbole.

If Rowling were a member of a nationalist party, I suspect that one might be able to get away with calling her a Nazi.

Note the SNP aren't going around litigating against people calling them Nazis, because Nationalism and Socialism are literally their party manifesto, and the last thing they'd want in a newspaper is a court transcript of two lawyers arguing about how accurate it is to call them Nazis.